Apparatus and process for washing a pulp web



July 15, 1969 l.. M. SUTHERLAND APPARATUS AND PROCESS FOR WASHING A PULP WEB Filed Sept. 22, 1965 July 15, 1969 L M. sun-IERLAND A 3,454,970

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L. M. SUTHERLAND APPARATUS AND PROCESS POR WASHING A PULP WEB July 15, 1969 4 Sheets-Sheet I5 F-iled Sept. 22, 1965 O ||I o Q IINH July 15, 1969 l.. M. SUTHERLAND 3,454,970

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United States Patent O 3,454,970 APPARATUS AND PROCESS FOR WASHING A PULP WEB Lionel M. Sutherland, 291 River Drive, Tequesta, Fla. 33458 Filed Sept. 22, 1965, Ser. No. 489,246 Int. Cl. D21c 9/02, 9/18 U.S. Cl. 8-156 10 Claims ABSTRACT OF THE DISCLOSURE Apparatus and process are disclosed for washing pulp wherein the pulp is deposited on a movable foraminous belt to form a pulp web which is passed through compression rolls arranged to contact the pulp to form a nip area. Immediately after said nip area, wash liquid distributing apparatus is positioned to direct a ow of liquid to the upper surface of the pulp web as it expands on emerging at the downstream side of the nip area. A further refinement is disclosed consisting of an enclosure for excluding air from the region of the compression rolls and the area contacted by the wash liquid.

This invention relates to a pulp washing process and apparatus constructed and arranged for carrying out such process.

In the manufacture of paper, paper board and similar products, it is customary to start with wood chips which are treated with a variety of chemicals in a digester under conditions of heat and pressure. The resulting product is a mixture of wood pulp and liquid containing residual chemical and dissolved woody materials, the liquid portion being commonly referred to in the art as black liquor. Separation of the pulp from the black liquor is normally carried out in a washing operation, after which the wash liquor is evaporated to recover the chemicals contained therein.

It has long been recognized that use of excessive quantities of water in the pulp washing operation leads to inefficiencies since the excess water must be evaporated in the recovery process or otherwise disposed of to avoid stream pollution. Various processes and apparatus have been proposed and used in the art for washing such Wood pulp material. One such apparatus in common use consists of a wire-covered segmented drum having means for applying a partial vacuum inside the drum. The drum is slowly rotated in a vat and the pulp to be washed passes over the outer surface while a stream of countercurrent water or weak black liquor is applied to the moving sheet of matted pulp. Generally, two or more stages are used and the pulp mat is broken up, repulped and agitated with dilution liquor before being picked up on the next drum. It has also been proposed to advance the unwashed pulp between two foraminous belts which pass between wash boxes located above and vacuum boxes positioned below the wash boxes. This system is disclosed in U.S. Patent 2,356,285 which issued to Edward T. Street on Aug. 22, 1944.

The difficulties inherent in such washing processes are avoided in the practice of the present invention by carrying out a series of washing steps when the pulp is temporarily in a favorable condition which makes it possible to achieve the required washing action without the necessity of a suction or vacuum action. As hereinafter described, this is accomplished by first putting the pulp under pressure between suitable rollers, then releasing the pressure on the pulp and simultaneously ooding the top of the expanding pulp mat with the desired quantity of very hot wash liquid. The black liquor in the puIp sheet is thereby displaced downward by the super- 3,454,970 Patented July 15, 1969 lICC imposed wash liquid before the sheet is again subjected to pressure. This results not only in a much improved washing action but also makes it possible to carry out the pulp washing operation using significantly lower amount of water per unit weight of pulp washed to the same degree of cleanliness. When a vacuum is employed, the temperature of the wash liquid must be kept below the flash point of steam which is usually about 180 F. By eliminating use o f suction or vacuum to remove liquor, wash liquor at va temperature of about 212 F. can be used, thus lowering the black liquor viscosity and mproving drainage of black liquor from the pulp.

Another type of washing device, which has been rarely used, is the screw press consisting of a screw or worm enclosed in a perforated barrel. Flights in the screw progressively decrease in volume until pressures approximating 400 lbs. per square inch are reached. Liquor expressed from the pulp mass is forced through perforations in the barrel into a storage system. A thickener or similar device and several screw presses are used to achieve results comparable to a vacuum system. Discharge consistency from a screw press is generally in the range of 35% to 40%. Countercurrent dilution, either water or weak black liquor, is added at the discharge from each press and mixed, as thoroughly as possible, with the pulp mass before entering the following screw press.

A preferred form of apparatus by which the process of the present invention may be practiced is diagrammatically illustrated in the accompanying drawings where- 1n:

FIG. l is a perspective view showing three pulp Washing units arranged in series and associated apparatus.

FIG. 2 is a cross-sectional view taken along the lines II-II of FIG. l.

FIG. 3 is a plan View taken as indicated by the arrows III-III of FIG. 2.

FIG. 4 is a diagrammatic view showing an alternate form of the present invention.

FIG. 5 shows an upper roll 19 with an abrasive cover.

FIG. 6 shows a lower roll with a mesh cover.

FIG. 7 shows a lower roll with narrow circumferential grooves to speed drainage of liquor from the nip area.

In the drawings there is shown in FIG. 1 in diagrammatic form a general arrangement of apparatus for carrying out the process of the present invention. In FIG. l the pulp from the digester, after diluting, screening `and hot brown stock refining or other customary treatment, is introduced at stock inlet 10 and passes through pipe 11 to thickener 12 where the density is raised to about 12% to 15% oven dry. This thickening step is carried out in thickener apparatus well known in this art and, therefore, the details of the thickener apparatus are not shown. After the thickening step, the thickened pulp is fed through 'a distributing box 13 to a moving foraminous belt 14 which is supported by rolls 15 and 16 to which it is held in frictional engagement lby tension and guide roll 17. In lieu of a thickener, an extended foraminous belt may be used with addition of more squeeze rolls and rolls or foils to support the substantial weight of the pulp slurry at low densities.

As shown in FIG, 2, the foraminous belt 14 and the loose mass of pulp 9 supported thereon pass through pairs of squeeze rolls and then through compression rolls 19 and 20 where the pulp sheet is compressed against the belt 14. Liquor expressed from the pulp by the high fluid-pressure gradient is collected below in pan 21. In excess of of the blackliquor present in the pulp delivered to the washing system is removed lby the combined effects of drainage and high pressure displacement lbefore the sheet emerges from the nip of the compression roll in the first stage washer and before dilution with any countercurrent wash water. Consequently black liquor returned to the recovery system is appreciably stronger (more concentrated) and contains a higher percent of black liquor solids than is possible with conventional washing systems and a substantial saving is effected in evaporating the excess water. As the compressed pulp emerges from the nip of the compression rolls 19 and 20, it undergoes a rapid expansion which takes place in a brief period as the pulp is leaving the nip of the rolls. Spray header 22 and its associated nozzles 23 are positioned so as to direct a film of weak black liquor or hot fresh water, as the case may be, under low pressure to immediately and uniformly cover the pulp sheet while it is undergoing expansion as it emerges from the nip of compression rolls 19 and 20. Alternatively, the stream of wash fiuid may be directed at the lower portion of the top roll. The force of gravity will then deliver the wash uid to the expanding pulp emerging at the nip area. The expanding pulp mat acts like a sponge to absorb the countercurrent wash liquor. The black liquor retained in the pulp after the first compression rolls, amounting to about one pound of liquor per one pound of pulp, being of higher specific gravity than the weaker countercurrent liquor is displaced downwardly and concentrated in the lower portions of the pulp mat by the superimposed wash liquor.

The foraminous belt may be elevated slightly as it nears the compression rolls and the center may also be crowned, if desired, to allow an excess liquor displaced on the top of the pulp ahead of the compression rolls to drain from the top of the partly compacted ribbon of pulp. Black liquor separated from the pulp is collected in pan 21.

In order to minimize the effects of foaming which frequently occurs under washing conditions due to admixture of air with foam forming materials in the black liquor, there is provided an enclosure 24 having an inlet 25 through which steam under pressure may -be introduced to exclude air from mixing with the black liquor. Foaming conditions are less bothersome after the strong and intermediate liquors have been removed so hoods may not be required after the first stage compression rolls.

The washed pulp sheet is carried from the first washing stage to the foraminous belt of the next washing stage where the ooding action, in the zone of pressure release, is repeated as shown in FIG. 1. Successive washing steps may be employed with the washed pulp being removed from the last stage for subsequent treatment, such as refining, bleaching or high density storage. Depending upon the next process operation the pulp may be, and generally would be, diluted with fresh or white water at the exit from the last stage washer nip.

Typical preferred conditions for carrying out the process of the present invention are set forth below by way of example.

Following the usual digesting and pulping process, pulp at about 11% density is blown from the digester into a blow tank. Black liquor is then added for dilution to permit screening to remove tramp metals, knots and oversized pieces of undigested wood. The hot brown stock may also be processed through refiners before washing. The density of the pulp at this point is from about 1.25% to about 4%. This initial concentration is raised to about 12-15 by the thickener, after which the thickened pulp is fed to a moving foraminous belt where it is spread, as evenly as possible, in the form of a loose mat having a compressed thickness of from .100 to to .250. The foraminous belt may be a fine mesh wire driven at a speed of up to about 500' per minute depending upon the thickness of the pulp mat when compressed and the drainage characteristics of the particular pulp. Alternatively, a foraminous belt of plastic, such as polyethylene or polypropylene may be used. The fine mesh material in contact with the pulp may be supported from below yby a heavy duty, coarse mesh belt of the same or different material.

After passing through squeeze rolls 18 where increasing pressure on the successive pairs of rolls removes nearly equal portions of residual black liquor at each position, the pulp is subjected to a considerably greater pressure at the nip of compression rolls 19 and 20. Where the density (oven or bone dry consistency, as it is frequently called) is of the order of 15% discharging from the thickener, it will rise to about 20% after the first pair of squeeze rolls and to about 30% after the second pair of squeeze rolls and will increase to 50% or higher discharging from the nip of the first pair of compression rolls. At this latter point, or more of the liquor content of the pulp slurry fed to the thickener will have been removed from the pulp prior to dilution with countercurrent wash liquor. Wetting the pulp mat upon emergence from the compression roll nip will be at the rate of one to three pounds of water per pound of pulp depending upon the concentration of black liquor required for efficient operation of the recovery system and cleanliness of the pulp needed in subsequent processing such as bleaching.

To accomplish the desired compression :and decompression, the compression rolls in each stage are loaded, preferably hydraulically, to maintain a uniform load of 300 to 1,000 pounds per lineal inch. Consequently, the density at the emergence from the compression rolls nip will be substantially the same at each stage.

Since the black liquor has a solids content much greater than the wash liquor, there is a concentration of black liquor toward the bottom of the sheet. Consequently, when expansion of the sheet takes place rapidly upon emergence from the nip of the compression rolls, the weight of the very hot water or weak black liquor flooding onto the expanding pulp sheet acts like a plunger to displace the black liquor of higher specific gravity downward through the sheet interstices into the lower portion thereof. In this way the strong black liquor is concentrated in the lower portion of the pulp mat and replaced by the superimposed countercurrent wash liquor. When pressure is again applied by the succeeding pair of cornpression rolls, the residual strong black liquor in the lower part of the mat is forced out of the mat first by the weaker liquor or hot water above. For this reason, mixing of the two concentrations of iiuid is to be avoided as much as possible. In all the other known processes countercurrent liquor, entr-aimed black liquor and pulp are deliberately mixed between each stage as a means of diluting the pulp prior to the next stage of washing and to improve washing by inducing diiusion of solids from the pulp and into solution or suspension in the liquid phase.

In order to accomplish optimum results at the compression rolls, atop roll having a pattern such as a herringbone, circumferential or longitudinal grooving, or alternately, a roll with an abrasive face or otherwise roughened surface, is desired to grasp the frequently slippery pulp. A suitable lower roll will have circumferential or spiral grooves to work in conjunction with the upper roll and provide channels to drain ol black liquor forced out of the pulp mat by the high fluid pressure gradient developed by compression roll pressure.

After the first press discharge, the mat has sufficient wet strength to support its own weight unless unusually large quantities of dilution fluid are added, for example, in excess of three pounds water per pound of pulp. When the mat can support its own weight, the foraminous belt should be eliminated. Alternatively, the lower roll may be covered with fine mesh material on the pulp side with a coarse mesh backing material for greater strength mounted against the roll. The tine mesh material may be from about 50 to 100 mesh. Liquor forced through the tine mesh cover enters the large interstices in the coarse mesh and drains into pan 21. Another method, though preferred the least, is to use a mesh cover over a drilled lower roll so liquor is forced into the lower compression roll interior and withdrawn to liquor storage by means of siphons located inside the roll.

In order to prevent foaming, steam under pressure is introduced into the enclosures 24 through inlets 25. This prevents air from entering the sheet during the rapid expansion which takes place immediately following the nip of the compression rolls and makes it possible for the wash liquor to fill the interstices in the sheet and exclude air. Mixtures of air and black liquor cause serious operating difficulties from foam and loss of valuable chemicals. The buildup of excessive pressure within the enclosures 24 is prevented by the relief valves 25a.

The drive means for the Various rolls are illustrated in FIG. 1 of the drawings in purely diagrammatic form. Gear drives for such rolls are well known and could be used to obtain rotation of the rolls in the directions indicated. The speed of roll rotation may be the same for each of the rolls or in some cases improved results may be obtained by providing a somewhat faster speed of rotation of the rolls 19 as compared to the speed of rotation of the rolls 20.

Flow of countercurrent Wash liquid should be metered to provide the desired amount of wash water at all times. The quantity of fresh water used can be adjusted to provide the necessary pulp cleanliness. In conjunction with pans 21, level or ow controls are desirable to maintain a steady flow of black liquor from the pans to storage tanks 26 and 27 and of wash liquor through the washing system.

While various combinations of rolls may be used in the practice of the present invention, I have illustrated preferred combinations of rolls in FIGS. 5 to 7. FIGURE 5 shows an upper roll 19 which has a cover 33 made up of an abrasive material. This roll may be used in conjunction with a lower roll of the type shown in FIG. 6 in which the roll cover consists of two overlying layers of mesh made of plastic or wire, the innermost layer 34 being coarse mesh and the outermost layer 35 being line mesh.

FIG. 7 shows a lower roll which has narrow circumferential grooves 36. These grooves operate to assist in rapid drainage of liquor from the nip tarea.

FIG. 4 shows an alternate form of the invention in which a head box 37 is employed in place of the thickener 12 and its distributing box 13. An elongated first stage is provided as shown in FIG. 4 in order to accomplish sufiicient black liquor removal from the mat prior to arrival of the mat at the irst pair of pressure rolls.

Having thus described my invention, I claim:

1. The process of supplying a slurry of digested wood pulp to an elongated foraminous belt, gradually removing the liquid and simultaneously increasing the density of the pulp mat, passing the pulp mat between compression rolls under high lineal pressure thereby forming a nip area, and then applying a quantity of wash liquid into the nip area on the downstream side of the compression rolls onto the pulp mat as it is expanding when the pressure is released therefrom on emergence from the nip area of the compression rolls.

2. The process of washing digested pulp which includes the steps of diluting the pulp after digestion, removing tramp metals, knots and undigested pieces of wood, increasing the density to about 12 to 15%, distributing the thickened pulp evenly on a moving foraminous belt, thereby forming a loose mat of pulp, removing entrained black liquor from the pulp by steps in nearly equal portions, thereby increasing the pulp mat density, passing the pulp mat between compression rolls which contact the pulp under pressure and form a nip area, and then applying hot wash liquor into the nip area on the downstream side of the compression rolls onto the pulp mat as it expands upon emerging from the nip area of the compression rolls.

3. The process of claim 2 further characterized by the step of displacing the air which normally would be present in the washing area with steam.

4. Pulp washing apparatus comprising a foraminous belt, pulp distributing means positioned for depositing a slurry of wood pulp to form a pulp web on the upper surface of said belt, means for advancing the foraminous belt, at least one pair of compression rolls positioned above and below said belt in pressure contact with said pulp web thereby forming a nip area, and means for distributing wash liquid into the nip area on the downstream side of said rolls.

5. Washing apparatus according to claim 4 wherein there are a plurality of washing stages each comprising compression rolls in contact with the pulp web and forming a nip area, and washing means positioned to deliver wash liquid evenly on the pulp in each washing stage in the nip area on the downstream side of the compression rolls.

6. The apparatus of claim 4, wherein said means for advancing includes drive means for rotating the compression rolls at different speeds relative to each other, such that the upper roll rotates at a faster speed than the lower roll.

7. The washing apparatus of claim 4, further characterized by drive means for rotating each of the compression rolls.

8. The washing apparatus of claim 4 including rolls for supporting and guiding said foraminous belt; said advancing means including means for driving the belt in the form of a continuous loop; also including collecting means for receiving liquid displaced from the pulp.

9. The washing apparatus of claim 8 further characterized by the fact that said collecting means is positioned within the continuous loop formed by the foraminous belt.

10. Pulp washing apparatus comprising a foraminous belt, pulp distributing means positioned for depositing a slurry of wood pulp to form a pulp web on the upper surface of said belt, means for advancing the foraminous belt, at least one pair of compression rolls positioned above and below said belt in pressure contact with said pulp web thereby forming a nip area, and means for distributing wash liquid into the nip area on the downstream side of said rolls, further characterized by an enclosure for excluding air from the region of the compression rolls and the washing liquid contact area at the place of emergence of the pulp from the nip area of the compression rolls, and means for introducing steam under pressure into said enclosure.

References Cited UNITED STATES PATENTS 2,356,285 8/1944 Street 162-301 2,772,138 1l/1956 Evans 8-156 X 2,803,125 8./1957 Mesek 8-156 X 2,970,882 2/1961 Kumin et al. 8-156 X FOREIGN PATENTS 193,277 ll/ 1957 Germany.

S. LEON BASHORE, Primary Examiner U.S. C1. X.R. 

